The nature of cellular and humoral in vitro reactions is being investigated in BALB/c mice implanted with syngeneic mammary tumors. Three different lines of this mouse strain are used: (1)\BALB/cCrgl, which has a low incidence of spontaneous mammary tumors (SMT) and is devoid of intact mouse mammary tumor virus (MMTV); (2)\BALB/cfC3H with a high incidence of SMT and expression of complete MMTV particles; and (3)\hereditarily asplenic BALB/c mice heterozygous for the nude gene, which have a high incidence of SMT appearing at an early age. The main goal is the identification, characterization and isolation of the immunologically relevant cells. Adoptive transfer experiments will be performed to assess the possible in vivo immunotherapeutic value of the lymphoid cells known to be of importance in in vitro reactions.